Improving thermal stability of a microcavity emitter for utilization under atmospheric environment

With the development of micro-fabrication technology, various metamaterials with controlled emission spectra have been proposed as thermal emitters. However, general metamaterials have a risk of deformations and degradation at high temperatures in atmospheric conditions, which is inconvenient for use as a thermal emitter. In this study, we propose a concept to enhance the thermal durability of microcavity-type metamaterials. Although typical microcavities are entirely composed of metal to excite the resonance of electromagnetic waves, we assessed the feasibility of a microcavity consisting of silicon with minimal metal coatings. While usual metals are oxidized at high temperatures, gold is rarely oxidized due to its chemical stability. However, the gold layer deposited on the Si substrate has the potential to melt below 400 °C due to the formation of an Au-Si eutectic alloy, which has a much lower melting point than pure gold. Therefore, we focused on the gold-tungsten bilayer as a suitable metal coating for the silicon microcavity, thereby preventing oxidation and melting that would otherwise influence the emission spectra of the thermal emitter. The numerical analysis ensured that the proposed microcavity exhibited electromagnetic resonance, similar to that of a microcavity entirely composed of metal, unless the metal coating was too thin. The fabricated microcavity with the gold-tungsten coating also exhibited a thermal emission within a limited wavelength range, due to the microcavity resonance. Moreover, the heating experiment revealed that the microcavity with a gold-tungsten coating maintained its emissivity even when heated to 400 °C, which is higher than the oxidation point of tungsten and the melting point of the Au-Si eutectic alloy. Consequently, the gold-tungsten coating would be a reasonable approach to improve the stability of the microcavity-type metamaterial at high temperatures under oxidative conditions.